pterrordax hatchling and voodoo figurine summoning spells have similar flavor text as other rare archaeology items. it seems very likely that these two companions will be discovered through this profession. possibly fossil and troll digsites?
there's already speculation on wowhead's PTR site that the guardian cub will be a small mini-pet version of the new mount that blizzard announced it would be releasing as a blizzard store item in the near future. we will have to wait and see on this one.
last but not least, blizzard released an official preview of some of the new pets coming in patch 4.1. going from left to right - lashtail hatchling, nightsaber cub, winterspring cub, panther cub, legs.
in light of this preview, there are a couple things that are highly likely:
1. patch 4.1 will be released before children's week (before may 1st)
2. the lashtail hatchling will indeed be available as a pet (and she'll look slightly different than the razasshi hatchling, yay!)
3. YES, legs will be a baby marshwalker